Originally Posted by
Teufel Racing
Monty,
Here's a little challenge for ya. That is if you would care to accecpt it. LOL.
Head shims? What is the general rule of thumb for re shimming a motor as it gets older or when shimming for fuel. I.E., 20%, 30%, or even 40% nitro. It's something that gets talked about but not really explained in detail as to why you would or wouldn't add or remove a head shim(s). This is of course not something that many people would do and probably something that maybe they shouldn't do. However, should the need arise on race day, what are the in's and out's to do it right. Sorry to put you on the spot and maybe I'll regret asking, but I know I don't know much about it and it seems that it can be almost as important as running the right pipe and header combo.
The challenge is in the amount of typing it would take to answer in uber detail
Basic gist , raising and lowering compression , lower equals more, higher equals less, this also changes the ignition timing since our engines run on shear compression to make fire, the plug plays a large factor in this also, if you shim down use a cooler plug.
Higher nitro you typically want less compression as the fuel is more explosive and might possibly predetonate if the shimming is too low , vice versa for lower content nitro.
If you need to gain some compression cause the p/s is getting a little worn than in a pinch you can lower the shimming but I wouldn't go any lower than .6 mm with 30%fuel
Hope this helps
Hope I passed
rule of thumb , hmmmm if it feels weak on the low end even after you changed the pipe or plug or clutch , etc. , try to shim it down in small increments until you get the feel you are looking for , try .05mm increments